Various chemical precipitation methods have been employed as one aspect of a complete system for the removal of heavy metal pollutants from aqueous solutions. Sulfide precipitation is often employed because sulfide salts may be used to remove heavy metal pollutants such as lead, copper, silver, cadmium, zinc, mercury, and nickel. Sulfides are also used as a reducing agent to convert hexavalent chromium to trivalent chromium. There are two basic types of sulfide precipitation processes—the soluble sulfide process and the insoluble sulfide process.
By the soluble sulfide process, a soluble sulfide salt such as sodium sulfide or sodium hydrosulfide is added to a wastewater stream containing at least one heavy metal pollutant. The soluble salt quickly dissociates into sodium ions and sulfide ions and the heavy metal pollutant ions rapidly react with the sulfide ions to form a relatively insoluble heavy metal salt, which precipitates out of solution. There are two common problems associated with the soluble sulfide process. First, the relatively insoluble heavy metal pollutant sulfide salt often forms as very fine colloidal particles, which are not easily filtered or otherwise separated from the wastewater stream. Of even greater concern is the formation of odorous and highly toxic hydrogen sulfide gas, which invariably results from the high concentration of soluble sulfides present in the soluble sulfide process. Thus, the soluble sulfide process must be carefully monitored and controlled in order to avoid discomfort and harm to the treatment personnel.
Sulfur dioxide and sodium metabisulfite are used for chromium reduction, and again, close pH control is necessary to balance the efficiency of use, and the evolution of corrosive and toxic sulfur dioxide gas.
Ferrous sulfate and ferrous chloride are used alone and in conjunction with sodium metabisulfite. The same concerns are present with sulfur dioxide gas evolution, and the iron salts generate unacceptable amounts of sludge, which is considered a hazardous waste.
U.S. Pat. No. 3,740,331 represents an early attempt to exploit the benefits of the soluble sulfide process while avoiding the formation of hydrogen sulfide gas. A soluble heavy metal salt was added either immediately after or immediately before the addition of the soluble sulfide salt to the wastewater stream. The heavy metal of the soluble heavy metal salt was chosen based on its relative equilibrium sulfide ion concentration as compared to that of the given pollutant heavy metal. That is, the slightly less insoluble heavy metal ion of the soluble heavy metal salt was added to the wastewater stream during the soluble sulfide process to act as a scavenger for excess sulfide thereby avoiding the formation of H2S.
The insoluble sulfide process is a variation on the chemistry disclosed in the '331 patent. By the insoluble sulfide process, a freshly prepared slurry of an essentially insoluble heavy metal sulfide salt is added to a wastewater stream. Hereagain, the heavy metal of the essentially insoluble heavy metal sulfide salt is chosen based on its relative equilibrium sulfide ion concentration as compared to that of the given pollutant heavy metal. Specifically, the essentially insoluble heavy metal sulfide salt must be slightly less insoluble than the heavy metal pollutant salt, which will eventually be formed. Thus, as the essentially insoluble heavy metal sulfide salt dissociates in solution, the heavy metal pollutant salt is formed. The essentially insoluble heavy metal sulfide salt can only further dissociate as the sulfide ions are consumed in the formation of the heavy metal pollutant salt. Therefore, there is never an excess of sulfide ions such that H2S formation is avoided.
U.S. Pat. No. 4,102,784 discloses an insoluble sulfide process, which is concerned with avoiding the formation of very fine, colloidal particles of the resultant heavy metal pollutant salt. By the process set forth in the '784 patent approximately 90% of the insoluble sulfide particles must have a diameter of at least 50 microns or more. The '784 patent also requires that the insoluble sulfide slurry is maintained at a pH of greater than 7 in order to avoid H2S formation. Further, although the slurry disclosed in the '784 patent is formed in the absence of pollutant heavy metal ions to avoid the production of colloidal particles such as that patent asserts are formed in accordance with the method of the '331 patent, discussed above, the slurry of the '784 patent is, nevertheless, formed on site at the wastewater treatment facility and must be constantly, carefully agitated and temperature controlled prior to addition to the wastewater stream. Because the slurry is formed on-site, it must be maintained and fed to the wastewater stream at a pH of greater than 7 in order to reduce the formation of hydrogen sulfide gas which will form in the presence of any excess sulfide ions.
U.S. Pat. No. 4,422,943 discloses an insoluble sulfide process, which employs iron pyrite, FeS2, rather than a ferrous or ferric sulfide slurry. That process requires that the slurry is made on-site at the wastewater treatment facility and is added to the wastewater stream at a pH above 7 in order to avoid the formation of H2S gas, which will form in the presence of any excess sulfide ions.